Crack Paths 2012

Experimental investigations on mixed-mode-loaded cracks

H. A. Richard1, N.-H. Schirmeisen2, A. Eberlein1

1 Institute of Applied Mechanics, University of Paderborn, Pohlweg 47-49,

33098 Paderborn, Germany, e-mail: richard@fam.upb.de

2 Windmoeller & Hoelscher KG, Münsterstr. 50, 49525 Lengerich, Germany,

e-mail: nils-henrik.schirmeisen@wuh-group.de

ABSTRACT.Failure of safety-relevant parts is often caused by a superposition of

normal stress as well as plane and anti-plane shear stress. The loading cases of cracks

(Mode I, ModeII and ModeIII) are generally defined by the stress fields near the crack

tip. In contrast to pure ModeI-loaded cracks, whose stress near fields are symmetric,

the stress fields near the crack tip of mixed-mode-crack problems are unsymmetrical.

Consequently the fracture mechanical treatment of those mixed-mode-loaded cracks is

more complicated as of pure ModeI-cracks. This paper deals with experimental

investigations of crack growth under spatial mixed-mode-loading. In the following

detailed results of fracture and fatigue crack growth experiments and their comparison

with existing criteria are presented and discussed.

I N T R O D U C T I O N

In fracture mechanics it is important to understand and analyse the behaviour of mixed

mode fracture, because materials often contain different defects, e.g. pre-cracks, which

may have been introduced unintentionally during the manufacturing process. So pre

cracks can have an arbitrary orientation with respect to a general type of loading, which

a component of a machine or structure has to carry.

Cracks in structures or materials are generally divided into three fracture modes,

which are shown schematically in Figure 1a. The difference of the three fracture modes

is the orientation of the local stress filed near the crack tip. A ModeI-loading induces a

crack-opening; a ModeII-loading leads to an in-plane fracture of the material, which

causes also a crack kinking and a ModeIII-loading leads to an anti-plane fracture of the

material, which causes a crack twisting (Figure 1b).

Local mixed-mode-loading conditions at cracks occur in combination of the three

basic fracture modes. Thereby the cracks grow in a way, that not only an opening, but

also a planar deflection and/or a non-planar twisting of the two crack surfaces can be

found.

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